Electrolytes, supporting

Boujlel and Simonet used an electrochemical method to prepare a group of similar compounds, including compound ]5, shown in Eq. (3.41). In a typical case, benzil was reduced in DMF solution at the dropping mercury electrode in the presence of tetrabutylammonium iodide, used in this case as a supporting electrolyte rather than phase transfer catalyst. In the presence of diethylene glycol ditosylate, compound 15 (mp 77— 78°) was isolated in 10% yield. Using the same approach, acenaphthenedione was reduc-tively cyclized with triethylene glycol ditosylate to afford the product (mp 84—85°, 42% yield) shown in Eq. (3.42). 

However, in the case of stress-corrosion cracking of mild steel in some solutions, the potential band within which cracking occurs can be very narrow and an accurately known reference potential is required. A reference half cell of the calomel or mercury/mercurous sulphate type is therefore used with a liquid/liquid junction to separate the half-cell support electrolyte from the process fluid. The connections from the plant equipment and reference electrode are made to an impedance converter which ensures that only tiny currents flow in the circuit, thus causing the minimum polarisation of the reference electrode. The signal is then amplifled and displayed on a digital voltmeter or recorder. 

The conductivity of liquid and glass membranes is determined by ion-migration (absence of an excess of supporting electrolyte is assumed) in the diffusion layer. Equation (25) should then be written as ... 

Supporting electrolyte. Prepare a supporting electrolyte composed of l.OOM pyridine and 0.50M chloride ion, adjusted to a pH of 7.0 0.2 for use with a silver anode, or LOOM pyridine, 0.30M chloride ion and 0.20M hydrazinium sulphate, adjusted to a pH of 7.0 0.2, for use with a platinum cathode. A small background current is obtained with the latter. 

Reagents. Supporting electrolyte. Prepare a 0.1M phosphate buffer of pH = 8 containing 0.1M potassium iodide and 0.025 M potassium tartrate (0.17 g Na2HP04,12H20, 3.38 g NaH2P04,2H20, 4.15 g KI and 1.5 g potassium tartrate, in 250 mL of water). 

Procedure. Place 45 mL of the supporting electrolyte in the cell and fill the isolated cathode compartment with the same solution to a level well above that in the cell. Pipette 5.00, 10.00, or 15.00 mL of the 0.01 M antimony solution into the cell and titrate coulometrically with a current of 40 milliamps. Stir the solution continuously by means of the magnetic stirrer and take e.m.f. readings of the Pt-S.C.E. electrode combination at suitable time intervals the readings may be somewhat erratic initially, but become steady and reproducible after about 3 minutes. Evaluate the end point of the titration from the graph of e.m.f. vs counter reading this shows a marked change of e.m.f. at the end point. If it proves difficult to locate the end point precisely, recourse may be made to the first- and second-differential plots. 

Reagents. Supporting electrolyte. Prepare 0.2M potassium bromide from the analytical grade salt. 

Reagents. Supporting electrolyte. For chloride and bromide, use 0.5 M perchloric acid. For iodide, use 0.1M perchloric acid plus 0.4M potassium nitrate. It is recommended that a stock solution of about five times the above concentrations be prepared (2.5M perchloric acid for chloride and bromide 0.5M perchloric acid + 2.0A f potassium nitrate for iodide), and dilution to be effected in the cell according to the volume of test solution used. The reagents must be chloride-free. 

Catholyte. The electrolyte in the isolated cathode compartment may be either the same supporting electrolyte as in the cell or 0.1 M sulphuric acid the formation of mercury(I) sulphate causes no difficulty. 

The supporting electrolyte may be 0.5 M potassium nitrate for bromide and iodide for chloride, 0.5 M potassium nitrate in 25-50 per cent ethanol must be used because of the appreciable solubility of silver chloride in water. 

Reagent. Supporting electrolyte. Prepare a 0.5M potassium nitrate from the pure salt for chloride determinations the solution must be prepared with a mixture of equal volumes of distilled water and ethanol. 

Procedure. Place 50 mL of the supporting electrolyte in the coulometric cell, and pass nitrogen through the solution until a pH of 7.0 is attained thenceforth pass nitrogen over the surface of the solution. Pipette 10.00 mL of the acid into the cell. Adjust the current to a suitable value (40 or 20 mA), then start the electrolysis stop the titration when the equivalence point pH (7.00) is reached. 

Reagent. Supporting electrolyte. Prepare a 0.05 M sodium bromide solution. 

Procedure. Place 50 mL of the supporting electrolyte in the beaker and add some of the same solution to the tube carrying the silver electrode so that the liquid level in this tube is just above the beaker. Pass nitrogen into the solution until the pH is 7.0. Pipette 10.00 mL of either 0.01 M or 0.001 M hydrochloric acid into the cell. Continue the passage of nitrogen. Proceed with the titration as described under (a) above. 

Several successive samples may be titrated without renewing the supporting electrolyte. 

Procedure. Experience in this titration may be acquired by titration of, say, 5.00 mL of accurately standardised 0.01 M sodium hydroxide solution. Use 50 mL of supporting electrolyte and a current of 30 mA. 

The shift of the half-wave potentials of metal ions by complexation is of value in polarographic analysis to eliminate the interfering effect of one metal upon another, and to promote sufficient separation of the waves of metals in mixtures to make possible their simultaneous determination. Thus, in the analysis of copper-base alloys for nickel, lead, etc., the reduction wave of copper(II) ions in most supporting electrolytes precedes that of the other metals and swamps those of the other metals present by using a cyanide supporting electrolyte, the copper is converted into the difficultly reducible cyanocuprate(I) ion and, in such a medium, nickel, lead, etc., can be determined. 

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